The basic concepts of psychoanalytic psychology have been criticized from time to time on the grounds that they are couched in terms that lack scientific precision. That these terms have survived in spite of ambiguity and inconsistency is a tribute to their clinical utility and to the fact that the underlying propositions that they reflect are sound. It seems reasonable, however, to concur with the author's contention that further growth in psychoanalytic theory demands that these ambiguities of terminology be overcome. The author reexamines some of the Freudian terms relating to psychic energy and the structure of thought and attempts to reformulate them in terms of a mechanical model of the psychic apparatus of his own invention. Recalling first the extent to which science in general has benefited from such theoretical constructs, he then turns to psychoanalysis in particular and discusses the limitations of previous models of the psychic apparatus